The present invention relates to new and improved thermoelectric devices and to methods for manufacturing the same. In particular, this invention relates to such devices and methods in which the bonding structures between the individual thermoelectric elements and their adjacent conducting elements are optimized, resulting in compatibility of their properties. Characteristics of the thermoelectric elements, the conducting elements, and the bond structures therebetween are greatly improved. As a result, thermoelectric devices embodying this invention have enhanced operating efficiencies and longer operating lives.
Alternative energy sources are becoming increasingly important as our supply of fossil fuels diminishes. A partial solution to the problem of dwindling resources lies in energy conservation and full utilization of alternative resources. One way to more efficiently utilize our resources is to cut down on waste both by using our waste heat to generate electricity, and by more efficiently cooling and heating. Vehicles, power generation plants, and many industrial processes are inherently wasteful of heat by their very nature. Capturing and recycling some of this energy can avoid this waste. Thermoelectric devices can provide this generation of electricity from heat by direct conversion means, thereby decreasing waste and increasing efficiency. They also are capable of efficient cooling or heating by generating a thermal gradient from a d.c. current, via the Peltier effect.
The basic theory of thermoelectric effects was developed more than 100 years ago. The circuit for a simple thermoelectric device includes two dissimilar materials, for example, an n-type and a p-type thermoelectric semiconductor element and electrically conductive elements which are joined at opposite ends of the thermoelectric elements. In the most basic form, the thermoelectric elements are arranged in an alternating n-element:p-element configuration. They are joined alternatively top and bottom by the electrically conducting elements such that the electricity flows through the device. Each element then has two junctions with conducting elements at either end of the thermoelectric element, and these junctions are generally made with bonding structures. They may also be physically pressed together with a pressure contact system such as a spring, or other mechanical pressure configuration, however, these physical techniques have not been satisfactory from a contact resistance and integrity standpoint.
For power generation a temperature gradient is applied across the device. Heat may be applied to the hot side junction, at temperature T.sub.h, from an external source, while the cold side (T.sub.c) junction may be maintained at a lower temperature, usually by water cooling jackets or air circulation. As a result of the temperature difference, T.sub.h -T.sub.c, a current flows through the device in a series or parallel fashion depending on the circuit design. For cooling or heating, current is applied to the circuit and this current causes the pumping of heat from one side of the device to the other. Conversion from cooling to heating is achieved by reversing the direction of the current flow, whereby the junction that formerly was the cold side becomes the hot side.
The thermoelectric efficiency of a material or device is measured by the figure-of-merit (Z). It is dependent solely upon the properties of the materials, and is one indicia of the performance of a thermoelectric device. Z is defined as: ##EQU1## Where: Z is expressed in units.times.10.sup.3 /.degree.K.
S is the Seebeck coefficient in .mu.V/.degree.C. PA1 K is the thermal conductivity in mW/cm-.degree.C. PA1 .sigma. is the electrical conductivity in (.OMEGA.cm).sup.-1
As can be determined from the above equation, the highest value of Z occurs when the Seebeck coefficients(S) is high, the electrical conductivity (.sigma.) is high, and the thermal conductivity (K) is low.
The maximization of S.sup.2 .sigma. occurs when its carrier concentration is in the range of 10.sup.19 to 10.sup.20 extrinsic charge carriers per cubic centimeter. For this reason, semiconductors have a larger figure-of-merit than either metals or insulators. To minimize the thermal conductivity (K) and consequently increase the figure-of-merit, one must minimize the main components of K: K.sub.1, the lattice thermal conductivity, and K.sub.e, the electronic thermal conductivity. The lattice thermal conductivity comes about from phonon transport through the thermoelectric material. This factor stays relatively constant throughout the full range of extrinsic charge carrier concentrations for a semiconductor. The electronic thermal conductivity comes from the movement of electrons through the material. As the material increases in number of charge carriers, and thus becomes more metal-like, the electronic thermal conductivity takes on a greater value and plays the major role in its contribution to the overall value of K. K.sub.1 is the dominant factor for K in insulators and most semiconductors, while K.sub.e is the dominant factor for K in conductors.
In research on semiconductors for thermoelectric effects, it has been determined that no one material could provide a satisfactory figure-of-merit over a sufficiently wide temperature range to have broad thermoelectric utility over a wide temperature range. Consequently, much attention has been directed toward developing materials with a high figure-of-merit over relatively narrow temperature ranges. Most of the materials researched in semiconductors were binary and ternary compound tellurides and their solid solution alloys. Significant systems with regard to demonstrating thermoelectric device feasibility are solid solution alloys of bismuth telluride. On the basis of figure-of-merit alone, the Z values of bismuth telluride alloys above 250.degree. C. are not high enough to be practical. On the other hand, germanium telluride, lead telluride and lead telluride-tin-telluride alloys exhibit high values of Z at temperatures as high as 700.degree. C. However, the thermal instabilities of these materials due to evaporation of tellurium, limits their usefulness to temperatures not significantly above 500.degree. C. Evaporation leads to a high resistance at the semiconductor-metal contacts and to a decrease in mechanical strength. This also leads to a need for encapsulization or the thermoelectric devices, so as to avoid sublimation of the tellurium. Nevertheless, for waste-heat situations which occur at hot-side temperatures between 100.degree. C. and 300.degree. C., the bismuth telluride alloys have a higher Z value and perform the best.
An example of bismuth telluride materials is discussed in co-pending U.S. patent application Ser. No. 412,306, filed Aug. 27, 1982, now U.S. Pat. No. 4,447,277, entitled "New Multiphase Thermoelectric Alloys And Method Of Making Same" by Tumkur Jayadev and On Van Nguyen, which is incorporated herein by reference.
Thermoelectric devices include one or more modules comprising a plurality of individual thermoelectric elements which are electrically connected to each other in series and/or parallel combinations to form complete modules. The application will determine whether a module is connected in a series or parallel combination. Such factors as required power output, voltage and current requirements, electrical resistance and thermal conductivity are considered. Each of the individual thermoelectric elements has two separate junctions with each of two electrically conductive elements which form a part of the overall circuit connection.
The junctions between the individual thermoelectric elements and the electrically conductive elements are usually formed by bonding the thermoelectric elements to the electrically conductive elements with a bonding structure. The overall characteristics and properties of the thermoelectric device can be greatly affected by the compatibility of the bonding structure with the individual thermoelectric elements and with the electrically conductive elements. The greatest problems with incompatibility between the bonding structure and the individual thermoelectric elements and/or the electrically conductive elements arise from diffusion of components into and out of the thermoelectric element and/or the conductive element. This diffusion may result in degradation of any or all of the elements in the bonding structures. Other compatability problems arise, for example, from mismatching of bonding structure melting temperature and device operating temperature; and mismatching of thermal expansion coefficients, which will ultimately cause the bonds to break due to thermal stress. Thermal stress occurs when one component expands to a different degree than an adjacent one upon heating thereby straining the bonding structure.
It is imperative that all components of a thermoelectric device have satisfactory electrical conductivity. If the electrical resistance of the bond structure is high, an undesirably high voltage drop and energy loss results. High electrical conductivity of all components, including the bonding structure, is important to minimize this effect. Otherwise, the cumulative effect of the resistance of the bonding structures may reduce the performance of the device to an unacceptable level.
In addition to high electrical conductivity, the bonding structure must also exhibit high thermal conductivity to achieve a desirable performance. Operation of a thermoelectric device depends on maintaining a temperature gradient across the thermoelectric elements; therefore the thermal conductivity of the bond structure must be high to insure maximum temperature gradient across the semiconductor elements. Consequently, a bonding structure must have a high thermal conductivity to be compatible and enhance overall performance of a device.
It has been found in some applications that the performance and operating life of a thermoelectric device can be seriously limited by diffusion of materials from, to, and through the bond structure if the materials poison the thermoelectric elements or cause deterioration or breaking of the bond structures. The bond structures therefore forms an integral and important part of the overall thermoelectric device structure and has significant impact not only on device performance, but on the operating life of the device as well. An example of the diffusion problem arises when a conventional lead-tin bonding structure is used to bond nickel-plated electrically conductive copper straps to bismuth/antimony telluride pressed powder thermoelectric elements. The tin from the lead-tin bonding structure diffuses into the bismuth telluride thermoelectric element across the interface. Simultaneously, antimony diffuses out of the element into the bonding structure and turns the bond into a powdery substance which ultimately results in a failure of the bond and consequent loss of necessary electrical continuity. This problem occurs to a greater extent at the hot side junction simply due to its higher temperature which accelerates the diffusion rate. As a result, any bonding structure containing tin has been incompatible with the bismuth telluride system. It would be desirable to have a compatible bonding structure which could satisfy the thermoelectric and adhesion requirements above without creating serious diffusion problems.